<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .haha{
            width: 100px;
            height: 100px;
            border: 1px red solid;
        }
    </style>
</head>
<body>
    <div id="app">
        <div @mouseover="info=1" :class="{haha:info===1}">1</div>
        <div @mouseover="info=2" :class="{haha:info===2}">2</div>
        <div @mouseover="info=3" :class="{haha:info===3}">3</div>
    </div>


    <script src="../node_modules/vue/dist/vue.js"></script>
    <script type="text/javascript" >
        let vm = new Vue({
            el:"#app",
            data:{
                info:1
            }
        })
    </script>
</body>
</html>